A nano-sized tubular aluminum silicate is naturally produced as an imogolite, and an imogolite exists in soil, and is primarily produced in soil deriving from volcanic ash. Moreover, a natural imogolite, together with allophane as the related mineral, affects the transfer of nutrients and water in the soil and their supply to plants, and also affects the accumulation and residue of hazardous contaminated substances. The foregoing tubular aluminum silicate has, as its primary constituent elements, silicon (Si), aluminum (Al), oxygen (O) and hydrogen (H), and is a hydrated aluminum silicate assembled from numerous Si—O—Al bonds of a tubular shape having an outer diameter of 2.2 to 2.8 nm, an inner diameter of 0.5 to 1.2 nm, and a length of 10 nm to several μm, and is naturally a clay constituent distributed in the soil having as its base material ash fall ejecta such as volcanic ash and pumice.
A protoimogolite is a precursor substance of the imogolite, and becomes an imogolite by heating the precursor dispersed in an aqueous solution at approximately 100° C. Thus, the precursor substance during the process of forming the imogolite is referred to as a protoimogolite. Since a protoimogolite has properties that derive from the imogolite structure, it shows the same peak as imogolite at −78 ppm in a 29Si solid state NMR, and the silicon has a coordination of OH—Si—(OAl)3. Consequently, an imogolite and a protoimogolite have roughly the same adsorption behavior at a relative humidity of 20% or less even regarding water vapor adsorption characteristics, and, although the protoimogolite does grow to a relatively long tube shape as with a crystalline imogolite, it is considered to accordingly possess the imogolite structure. Accordingly, a protoimogolite possesses a similar adsorbent property as with an imogolite in a low-humidity range.
Meanwhile, a layered clay mineral also yields superior adsorption performance. Bentonite (mineral name of montmorillonite), which is swelling clay that is mainly used as a waterproofing agent, is known to well up to approximately 10 times relative to water adsorption. It is also known to possess a certain level of water vapor adsorption performance.
As described above, the unique shape and physical properties of an imogolite as the nano-sized tubular aluminum silicate and the layered clay mineral are considered to be industrially useful. In other words, since an imogolite and a layered clay mineral are characterized in being able to adsorb various substances based on their unique microstructure, for instance, it has been conventionally said that an imogolite and a layered clay mineral have application potency as a hazardous contaminant adsorbent, a deodorant, and the like. In addition, since an imogolite yields superior water vapor adsorption performance, its application as a heat exchange material for heat pump, a dew condensation prevention agent, an autonomous humidity control material and the like is also expected. Meanwhile, since a layered clay mineral that is known from the past also has water vapor adsorption performance, the improvement of such performance is expected.
In particular, since desiccant air conditioning aims to eliminate the moisture content in the air that is introduced from the outside air, it is required to efficiently eliminate the moisture content even from the high humidity air in the summer, and the adsorbent that is sought in the desiccant air conditioning is generally demanded of a high adsorption amount at a relative humidity of roughly 5% to 60%, but a higher adsorption performance in a high humidity range is also desirable.
Under the foregoing circumstances, the mass synthesis at an industrially inexpensive cost is being demanded while possessing the foregoing characteristics of the tubular aluminum silicate. Nevertheless, with only the amorphous substance including an imogolite structure that has been clarified with the NMR measurement, an adsorption performance that is approximately 1.2 to 1.5 times better in comparison to zeolite or silica gel could only be achieved. In giving consideration to the miniaturization of a desiccant air conditioner based on the foregoing background, the synthesis of an adsorbent having an adsorption performance that is twice or higher in comparison to existing materials is required.
Conventionally, adsorbents utilizing the unique fine pores of an amorphous substance utilizing an imogolite or a protoimogolite which becomes an imogolite when it is heated at approximately 100° C. have been developed (refer to Patent Documents 1 and 2). Nevertheless, with these conventional methods, it was not possible to achieve an adsorption performance of 45 wt % or more at a relative humidity of 60% in a water vapor adsorption isotherm.
In addition, adsorbents utilizing an amorphous aluminum silicate having a Si/Al ratio of 0.7 to 1.0, and peaks in the vicinity of −78 ppm and −87 ppm in a 29Si solid state NMR spectrum have been developed (refer to Patent Documents 3 and 4). Nevertheless, with these methods also, it was not possible to achieve an adsorption performance of 45 wt % or more at a relative humidity of 60% in a water vapor adsorption isotherm.
Meanwhile, among the technologies concerning the collection or separation of gases that are being developed pursuant to their objective and use, separation/collection technology of carbon dioxide is becoming an important subject from the perspectives including the issue of global warming.
In the methods of collecting carbon dioxide from dry gas, adsorbents of porous materials such as silica gel, alumina, zeolite, activated carbon, and diatomite, as well as silica, calcium oxide, sodium oxide, potassium oxide, iron oxide and the like are being used (refer to Patent Documents 5 and 6).
Moreover, with a majority of the pressure swing adsorption methods of carbon dioxide, zeolite 13X is being used as the adsorbent (refer to Non-patent Document 1). Zeolite 13X yields a superior adsorption amount of carbon dioxide in a range of roughly 0 to 2 atmospheres. Nevertheless, to desorb the adsorbed carbon dioxide for use as an effective pressure swing adsorbent, there is a problem in that vacuuming need to be performed and the pressure needs to be 0.2 atmospheres or less.    [Patent Document 1] Japanese Published Unexamined Application No. 2001-064010    [Patent Document 2] Japanese Published Unexamined Application No. 2006-240956    [Patent Document 3] Japanese Published Unexamined Application No. 2008-179533    [Patent Document 4] Japanese Patent Application No. 2008-224574    [Patent Document 5] Japanese Published Unexamined Application No. 2003-019435    [Patent Document 6] Japanese Published Unexamined Application No. 2005-040753    [Non-Patent Document 1] New Handbook on Adsorption Technology, published by NTS (1999) pages 133 to 140